INAA, IDMS AND SIMS COMPARATIVE REE INVESTIGATIONS OF CLINOPYROXENES FROM MANTLE XENOLITHS WITH DIFFERENT TEXTURES

Clinopyroxenes from mantle xenoliths from the Massif Central, France, have been chosen for a comparative study of rare-earth element (REE) a bundances obtained by means of conventional and in situ analyses. Xeno liths from Hungary and the Canary Islands showing unusual trace-elemen t or mineralogical features have also been investigated. Secondary ion mass spectrometry (SIMS) has been shown to be a powerful technique fo r both in situ and bulk investigations, competitive with instrumental neutron activation analysis (INAA) and isotope dilution mass spectrome try (IDMS) in the quantification of REE in clinopyroxenes. In addition , the application of SIMS for microspot analysis provides reliable inf ormation about REE concentrations with respect to texture. When SIMS i s applied for bulk investigations, its major advantages are the simple and fast sample preparation, and the small quantity of material requi red. The results obtained in this paper indicate that grain-by-grain a nd internal homogeneity of clinopyroxenes, which is related to texture , determines the degree of agreement between SIMS, INAA and ID. The da ta for clinopyroxenes from equigranular xenoliths, which recrystallize d to almost equilibrium conditions, show the best agreement. For clino pyroxenes from protogranular LREE-depleted xenoliths, the agreement is good when the whole clinopyroxene population has the same degree of d epletion, and the metasomatic La and Ce enrichment is negligible. When complex geochemical processes are investigated, such as subsolidus re actions and interactions with fluids and melts, conventional bulk tech niques are often inadequate and the SIMS technique is the only one cap able of thoroughly describing the effects of element migration at a mi croscale. For a proper understanding of geochemical processes in the m antle it is here suggested that SIMS investigations in thin section ar e made prior to mineral selection for subsequent elemental and isotopi c analysis by ID.